CRISPR is a term for Clustered, Regularly Interspaced Short Palindromic Repeats. A number of protein families appear only in association with these repeats and are designated Cas (CRISPR-Associated) proteins. This entry describes a conserved region of about 65 amino acids from an otherwise highly divergent protein found in a minority of CRISPR-associated protein regions. This region features two motifs of CXXC.